Friday, May 10, 2013

Chordates (phylum Chordata) are bilaterally symmetrical animals characterised by having a nerve cord within a flexible rod-shaped structure called a notochord running along their back. They also have pharyngeal slits, which are basically a series of openings that connect the inside of the throat to the outside, though in many chordates these are only present in the embryo. In additional, they have a post-anal tail, which is an extended structure beyond the anal opening, though again it may only be present in the embryo for many species.

There are three subphyla in this phylum as follow:

A) SUBPHYLUM TUNICATA

Tunicates (subphylum Tunicata) are chordates which mostly only have a notochord and a dorsal nerve cord in the larval stages. Adult tunicates are characterised by having a cellulose-like outer covering or "tunic" enclosing their body. They normally have two siphons - an incurrent siphon which takes in water, and an excurrent siphon which expels the water. In Singapore, they are represented by the salps and the ascidians.

Cephalochordates (subphylum Cephalochordata) are commonly referred to as lancelets or amphioxus. They appear fish-like, but unlike fishes they lack paired fins and only has a very poorly developed tail fin. Their nerve cord is within a simple notochord made up of tightly packed cells. They have no eyes, and use tentacle-like structures in front of the mouth to sense their surroundings and filter suspended food particles such as plankton from the water. They breathe through their skin, and hide in the sediment most of the time, though some of the specimens recorded from Singapore were caught with nets near the surface of the water. They are harvested and consumed in the region. The above photo features an unidentified museum specimen caught from the South China Sea.

C) SUBPHYLUM VERTEBRATA

Vertebrates are characterised by having a vertebral column (modified from the notochord in their embryo stage), which is a segmented series of stiff structures separated by mobile joints. Some species are known to retain their notochord till adulthood though. They are also the only group of chordates known to have a proper brain. Here are the classes of vertebrates that are recorded from Singapore:

Class Chondrichthyes

Cartilaginous fishes (class Chondrichthyes) are vertebrates characterised by having a skeleton made up of cartilage instead of bones. They also have tiny tooth-like scales (unlike the plate-like ones found on bony fishes) which make their outer coverings feel like sandpaper when touched. Most of them have limited physiological means to maintain the body temperature within a narrow range (i.e. they are ectothermic animals), but some species are adapted to generate heat and maintain a higher body temperature compared to the surrounding water.

Ray-finned fishes (class Actinopterygii) are ectothermic vertebrates characterised by their fins being supported by rays of bony/horny structures, much like those paper folding fans supported by rays of sticks. To make it easier for users to refer to, I have divided this class into two sections: i) Marine Ray-finned Fishes; and ii) Freshwater Ray-finned Fishes. Ray-finned fishes have bony skeletons (instead of cartilaginous ones). Most species have a layer of scales covering their bodies, while some may have very reduced scales or no scales at all.

Amphibians (class Amphibia) are ectothermic vertebrates thathave two distinctive stages of life - the juveniles are mostly aquatic and breathe with gills, until they undergo metamorphosis to develop lungs and (usually) limbs, and eventually become air-breathing adults. Most amphibians can breathe through their skin as well, and in fact, some species are known to rely solely on their skin for air exchanges. Their skin is not waterproof, and hence they must remain moist to prevent dehydration. They lay gelatinous eggs.

Reptiles (class Reptilia) are ectothermic vertebrates that are able to produce water-tight eggs. They are mostly covered in scales. Many reptiles have four legs, though the snakes and some lizards have no legs at all.

Birds (class Aves) are vertebrates characterised by the possession of feathers. Many of them have hollow bones, lacking marrow. Birds are endothermic, being able to generate body heat and maintain the body temperature within a narrow range. And like many of the reptiles and a few mammals, birds are able to produce water-tight eggs

Mammals (class Mammalia) are endothermic vertebrates characterised by the possession of hair on their body; the ability to produce milk in females to feed their young; and the possession of three middle ear bones. Most mammals give birth to live young, though a few species of extant mammals still produce water-tight egg.